Plasma processing apparatus and plasma processing method

ABSTRACT

An inductively coupled plasma process can effectively and properly control plasma density distribution within donut-shaped plasma in a processing chamber is provided. In an inductively coupled plasma processing apparatus, a RF antenna  54  disposed above a dielectric window  52  is segmented in a diametrical direction into an inner coil  58 , an intermediate coil  60 , and an outer coil  62  in order to generate inductively coupled plasma. Between a first node N A  and a second node N B  provided in high frequency transmission lines of the high frequency power supply unit  66 , a variable intermediate capacitor  86  and a variable outer capacitor  88  are electrically connected in series to the intermediate coil  60  and the outer coil  62 , respectively, and no reactance device is connected to the inner coil  58.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Japanese Patent Application No.2011-072582 filed on Mar. 29, 2011 and U.S. Provisional Application No.61/472,671 filed on Apr. 7, 2011, the entire disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to a technique for performing a plasmaprocess on a processing target substrate; and, more particularly, to aninductively coupled plasma processing apparatus and a plasma processingmethod.

BACKGROUND OF THE INVENTION

In a manufacturing process of a semiconductor device or a FPD (FlatPanel Display), plasma is used to perform a process, such as etching,deposition, oxidation or sputtering, so as to perform a good reaction ofa processing gas at a relatively low temperature. Conventionally, plasmagenerated by a high frequency electric discharge in MHz frequency bandhas been used in this kind of plasma process. The plasma generated bythe high frequency electric discharge is largely divided intocapacitively coupled plasma and inductively coupled plasma according toa plasma generation method (in view of an apparatus).

Generally, in an inductively coupled plasma processing apparatus, atleast a part (for example, a ceiling) of walls of a processing chambermay have a dielectric window, and a high frequency power is supplied toa coil-shaped RF antenna positioned at an outside of this dielectricwindow. The processing chamber serves as a depressurizable vacuumchamber, and a target substrate (for example, a semiconductor wafer anda glass substrate) is provided at a central region within the chamber. Aprocessing gas is supplied into a processing space formed between thedielectric window and the substrate. A high frequency AC magnetic fieldhaving magnetic force lines is generated around the RF antenna by a highfrequency current flowing in the RF antenna. The magnetic force lines ofthe high frequency AC magnetic field are transmitted to the processingspace within the chamber via the dielectric window. As the RF magneticfield of the high frequency AC magnetic field changes with time, aninductive electric field is generated in an azimuth direction within theprocessing space. Then, electrons accelerated by this inductive electricfield in the azimuth direction collide with molecules or atoms of theprocessing gas so as to be ionized. In this process, donut-shaped plasmamay be generated.

Since a large processing space is formed within the chamber, thedonut-shaped plasma can be diffused efficiently in all directions(particularly, in a radial direction) and a plasma, density on thesubstrate becomes very uniform. However, only with a conventional RFantenna, the plasma density on a substrate is not sufficiently uniformfor most plasma processes. In the plasma process, it is also one of theimportant issues to improve uniformity of a plasma density on asubstrate since a uniformity/reproducibility and a production yield of aplasma process depend on the plasma uniformity.

In the inductively coupled plasma processing apparatus, a characteristic(profile) of plasma density distribution within the donut-shaped plasmaformed in the vicinity of the dielectric window within the chamber isimportant. Especially, the profile of the plasma density distributionaffects characteristics (especially, uniformity) of plasma densitydistribution on the substrate after the diffusion of the plasma.

In this regard, there have been proposed several methods for improvinguniformity of plasma density distribution in a diametrical direction bydividing the RF antennal into a multiple number of circular ring-shapedcoils each having different diameter. There are two types of RF antennadivision methods: a first type of connecting the multiple number ofcircular ring-shaped coils in series (see, for example, PatentDocument 1) and a second type of connecting the multiple number ofcircular ring-shaped coils in parallel (see, for example, PatentDocument 2).

Patent Document 1: U.S. Pat. No. 5,800,619

Patent Document 1: U.S. Pat. No. 6,164,241

In accordance with the first type method among the aforementionedconventional RF antenna division methods, since an entire coil length ofthe RF antenna is large as a sum of all the coils, a voltage drop withinthe RF antenna may be fairly large and not negligible. Further, due to awavelength effect, a standing wave of electric current having a node inthe vicinity of a RF input terminal of the RF antenna may be easilyformed. For these reasons, in accordance with this first type method, itmay be difficult to achieve uniformity of plasma density distribution ina diametrical direction as well as in a circumferential direction. Thus,the first type method is essentially deemed to be inadequate for aplasma process for which plasma of a large diameter is necessary.

Meanwhile, in accordance with the second type method, the wavelengtheffect and the voltage drop within the RF antenna depend on a length ofeach of the coils segmented in parallel. Thus, the voltage drop withinthe antenna is relatively small. The second type method is advantageousin suppressing the wavelength effect. However, in the second typemethod, it is difficult to properly control current distribution withinthe RF antenna in a diametrical direction, and furthermore, the plasmadensity distribution right below the antenna.

Therefore, in the conventional plasma processing apparatus employing thesecond type method, variable capacitors for adjusting impedance areadditionally added (connected) to respective coils within the RF antennaso as to adjust a ratio of RF currents flowing through the respectivecoils. However, since the variable capacitors are highly expensive, itis not desirable in costs to use the variable capacitors for all coilswithin the antenna. As the number of the variable capacitors increases,electrostatic capacitances (parameters) to be adjusted increase. Thus,adjustment works are complicated.

Meanwhile, the conventional methods do not effectively overcome anundesired profile, where the plasma density becomes relatively higher ata central portion of the diametrical direction. Especially, in a lowpressure process, the plasma density may become easily higher at thecentral portion of the diametrical direction as a result of plasmadiffusion. This problem is not easily resolved by the conventionalmethods. In a large-diameter plasma processing apparatus, a differencein a coil diameter between an inner coil and an outer coil is large.Thus, the plasma density strongly tends to become relatively higher atthe central portion of the diametrical direction. The conventionalmethods could not have effectively achieved the uniformity of the plasmadensity distribution.

BRIEF SUMMARY OF THE INVENTION

In view of the above, the present illustrative embodiments provide aninductively coupled plasma processing apparatus and a plasma processingmethod, which are capable of effectively and easily controlling plasmadensity distribution within donut-shaped plasma in a diametricaldirection. Especially, it is possible to effectively and variouslycontrol the plasma density distribution in the diametrical direction tobe a proper profile when a pressure is low or plasma of a large diameteris necessary.

In accordance with one aspect of an illustrative embodiment, there isprovided a plasma processing apparatus. The plasma processing apparatusincludes a processing chamber having a dielectric window; a substrateholding unit for holding thereon a processing target substrate withinthe processing chamber; a processing gas supply unit configured tosupply a processing gas into the processing chamber in order to performa plasma process on the processing target substrate; an RF antennaprovided outside the dielectric window in order to generate plasma ofthe processing gas within the processing chamber by inductive coupling;and a high frequency power supply unit configured to supply a highfrequency power having a frequency for generating a high frequencyelectric discharge of the processing gas in the RF antenna. The RFantenna may include an inner coil, an intermediate coil, and an outercoil with gaps therebetween in a radial direction, respectively.Further, the inner coil, the intermediate coil and the outer coil may beelectrically connected to one another in parallel between a first nodeand a second node provided in high frequency transmission lines of thehigh frequency power supply unit. Furthermore, a variable intermediatecapacitor and variable outer capacitor may be provided between the firstnode and the second node, and the variable intermediate capacitor may beelectrically connected in series to the intermediate coil, and thevariable outer capacitor may be electrically connected in series to theouter coil. Moreover, no reactance device may be connected to the innercoil between the first node and the second node.

In accordance with another aspect of the illustrative embodiment, thereis provided a plasma processing apparatus. The plasma processingapparatus includes a processing chamber having a dielectric window; asubstrate holding unit for holding thereon a processing target substratewithin the processing chamber; a processing gas supply unit configuredto supply a processing gas into the processing chamber in order toperform a plasma process on the processing target substrate; an RFantenna provided outside the dielectric window in order to generateplasma of the processing gas within the processing chamber by inductivecoupling; and a high frequency power supply unit configured to supply ahigh frequency power having a frequency for generating a high frequencyelectric discharge of the processing gas in the RF antenna. The RFantenna may include an inner coil, an intermediate coil, and an outercoil with gaps therebetween in a radial direction, respectively.Further, the inner coil, the intermediate coil and the outer coil may beelectrically connected to one another in parallel between a first nodeand a second node provided in high frequency transmission lines of thehigh frequency power supply unit. Furthermore, a fixed or semi-fixedinner capacitor, a variable intermediate capacitor, and a variable outercapacitor may be provided between the first node and the second node,and the fixed or semi-fixed inner capacitor may be electricallyconnected to the inner coil. Moreover, the variable intermediatecapacitor may be electrically connected in series to the intermediatecoil, and the variable outer capacitor may be electrically connected inseries to the outer coil.

In accordance with still another aspect of the illustrative embodiment,there is provided a plasma processing apparatus. The plasma processingapparatus includes a processing chamber having a dielectric window; asubstrate holding unit for holding thereon a processing target substratewithin the processing chamber; a processing gas supply unit configuredto supply a processing gas into the processing chamber in order toperform a plasma process on the processing target substrate; an RFantenna provided outside the dielectric window in order to generateplasma of the processing gas within the processing chamber by inductivecoupling; and a high frequency power supply unit configured to supply ahigh frequency power having a frequency for generating a high frequencyelectric discharge of the processing gas in the RF antenna. The RFantenna may include an inner coil, an intermediate coil, and an outercoil with gaps therebetween in a radial direction, respectively.Further, the inner coil, the intermediate coil and the outer coil may beelectrically connected to one another in parallel between a first nodeand a second node provided in high frequency transmission lines of thehigh frequency power supply unit. Furthermore, a fixed or semi-fixedinner inductor, a variable intermediate capacitor, and a variable outercapacitor may be provided between the first node and the second node,and the fixed or semi-fixed inner inductor may be electrically connectedto the inner coil. Moreover, the variable intermediate capacitor may beelectrically connected in series to the intermediate coil, and thevariable outer capacitor is electrically connected in series to theouter coil.

In accordance with still another aspect of the illustrative embodiment,there is provided a plasma processing method for performing a plasmaprocess on a processing target substrate by using a plasma processingapparatus. The plasma processing apparatus includes a processing chamberhaving a dielectric window; a substrate holding unit for holding thereona processing target substrate within the processing chamber; aprocessing gas supply unit configured to supply a processing gas intothe processing chamber in order to perform a plasma process on theprocessing target substrate; an RF antenna provided outside thedielectric window in order to generate plasma of the processing gaswithin the processing chamber by inductive coupling; and a highfrequency power supply unit configured to supply a high, frequency powerhaving a frequency for generating a high frequency electric discharge ofthe processing gas in the RF antenna. The plasma processing methodincludes: segmenting the RF antenna into an inner coil, an intermediatecoil, and an outer coil with gaps therebetween in a radial direction,respectively, the inner coil, the intermediate coil and the outer coilbeing electrically connected to one another in parallel between a firstnode and a second node provided in high frequency transmission lines ofthe high frequency power supply unit; providing a variable intermediatecapacitor and a variable outer capacitor between the first node and thesecond node, the variable intermediate capacitor being electricallyconnected in series to the intermediate coil, the variable outercapacitor being electrically connected in series to the outer coil, noreactance device being connected to the inner coil; and controllingplasma density distribution on the substrate by selecting or variablyadjusting electrostatic capacitances of the intermediate capacitor andthe outer capacitor.

In the plasma processing apparatus or the plasma processing method, whena high frequency power is supplied from the high frequency power supplyunit to the RF antenna, an RF magnetic field is generated around each ofthe inner coil, the intermediate coil, and the outer coil of the RFantenna by high frequency currents flowing in the respective coils.Further, an inductive electric field configured to generate highfrequency electric discharge of the processing gas, i.e., donut-shapedplasma in the processing chamber is formed. The generated donut-shapedplasma is diffused in all directions within the processing chamber, sothat the plasma density on the substrate is uniformized.

When directions of the coil currents flowing in the inner coil, theintermediate coil, and the outer coil, respectively, are identical tothe circumferential direction, the plasma density within thedonut-shaped plasma has a maximum value at portions corresponding to therespective coils. In this case, by varying or adjusting electrostaticcapacitances of the intermediate capacitor and the outer capacitor, anintermediate coil current flowing in the intermediate coil and an outercoil current flowing in the outer coil are varied with respect to aninner coil current flowing in the inner coil. In this manner, it ispossible to control the plasma density distribution within thedonut-shaped plasma, and furthermore, the plasma density distribution onthe substrate.

Further, it is possible to flow the intermediate coil current or theouter coil current in a direction opposite to the other coil current byadjusting the electrostatic capacitance of the intermediate capacitor orthe electrostatic capacitance of the outer capacitor. In this case, itis possible to locally lower the plasma density at the portion in thedonut-shaped plasma, corresponding to the intermediate coil or the outercoil. Furthermore, a freedom degree of the plasma density distributioncontrol on the substrate can be expanded.

In accordance with still another aspect of the illustrative embodiment,there is provided a plasma processing apparatus. The plasma processingapparatus includes a processing chamber having a dielectric window; asubstrate holding unit for holding thereon a processing target substratewithin the processing chamber; a processing gas supply unit configuredto supply a processing gas into the processing chamber in order toperform a plasma process on the processing target substrate; an RFantenna provided outside the dielectric window in order to generateplasma of the processing gas within the processing chamber by inductivecoupling; and a high frequency power supply unit configured to supply ahigh frequency power having a frequency for generating a high frequencyelectric discharge of the processing gas in the RF antenna. The RFantenna may include an inner coil, an intermediate coil, and an outercoil with gaps therebetween in a radial direction, respectively.Further, the inner coil, the intermediate coil and the outer coil may beelectrically connected to one another in parallel between a first nodeand a second node provided in high frequency transmission lines of thehigh frequency power supply unit. Furthermore, a variable innercapacitor, a variable intermediate capacitor, and a fixed or semi-fixedouter capacitor may be provided between the first node and the secondnode, and the variable inner capacitor may be electrically connected tothe inner coil. Moreover, the variable intermediate capacitor may beelectrically connected in series to the intermediate coil, and the fixedor semi-fixed outer capacitor may be electrically connected in series tothe outer coil.

In the plasma processing apparatus, when a high frequency power issupplied from the high frequency power supply unit to the RF antenna, anRF magnetic field is generated around each of the inner coil, theintermediate coil, and the outer coil of the RF antenna by highfrequency currents flowing in the respective coils. Further, aninductive electric field configured to generate high frequency electricdischarge of the processing gas, i.e., donut-shaped plasma in theprocessing chamber is formed. The generated donut-shaped plasma isdiffused in all directions within the processing chamber, so that theplasma density on the substrate is uniformized.

When directions of the coil currents flowing in the inner coil, theintermediate coil, and the outer coil, respectively, are identical tothe circumferential direction, the plasma density within thedonut-shaped plasma has a maximum value at a portion corresponding toeach of the coils. In this case, by varying or adjusting electrostaticcapacitances of the intermediate capacitor and the inner capacitor, anintermediate coil current flowing in the intermediate coil and an innercoil current flowing in the inner coil, respectively, are varied withrespect to an outer coil current flowing in the outer coil. In thismanner, it is possible to control the plasma density distribution withinthe donut-shaped plasma, and furthermore, the plasma densitydistribution on the substrate.

Further, it is possible to flow the intermediate coil current in adirection opposite to the other coil currents by adjusting theelectrostatic capacitance of the intermediate capacitor. In this case,it is possible to locally lower the plasma density at a portion in thedonut-shaped plasma, corresponding to the intermediate coil or the outercoil. Furthermore, a freedom degree of the plasma density distributioncontrol on the substrate can be expanded.

In accordance with the plasma processing apparatus or the plasmaprocessing method of the illustrative embodiment, it is possible toeffectively and easily control the plasma density distribution withinthe donut-shaped plasma, with the above-described configuration andoperation. Especially, it is possible to effectively and variouslycontrol the plasma density distribution in the diametrical direction tohave a proper profile when a pressure is low or plasma of a largediameter is necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments will be described inconjunction with the accompanying drawings. Understanding that thesedrawings depict only several embodiments in accordance with thedisclosure and are, therefore, not to be intended to limit its scope,the disclosure will be described with specificity and detail through useof the accompanying drawings, in which:

FIG. 1 is a longitudinal cross sectional view showing a configuration ofan inductively coupled plasma processing apparatus in accordance with anexample of an illustrative embodiment;

FIG. 2 is a perspective view showing a basic configuration of a layoutand an electric connection of a variable capacitor provided within RFantenna in accordance with the illustrative embodiment;

FIG. 3 is a diagram illustrating operation of an experimental example ofFIG. 2;

FIG. 4 is a diagram showing electrical connection and an operation ofthe electrical connection when capacitors are omitted in the RF antennaof FIG. 2;

FIG. 5A is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna for anexperiment in accordance with the illustrative embodiment;

FIG. 5B is a diagram showing coil current distribution and electrondensity distribution measured by varying electrostatic capacitance of anintermediate capacitor in the above-described experiment;

FIG. 6A is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna in amodified example of the illustrative embodiment;

FIG. 6B is an explanatory view illustrating operation of theexperimental example of FIG. 6A;

FIG. 7A is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example;

FIG. 7B is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example;

FIG. 8 is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example;

FIG. 9 is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example;

FIG. 10 is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example;

FIG. 11 is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example; and

FIG. 12 is a diagram showing a basic configuration of a layout and anelectric connection of a variable capacitor added in RF antenna inanother experimental example.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, illustrative embodiments will be described with referenceto the accompanying drawings.

[Entire Configuration and Operation of Apparatus]

FIG. 1 illustrates a configuration of an inductively coupled plasmaprocessing apparatus in accordance with an illustrative embodiment.

The plasma processing apparatus is configured as an inductively coupledplasma etching apparatus using a planar coil RF antenna. By way ofexample, the plasma etching apparatus may include a cylindrical vacuumchamber (processing chamber) 10 made of metal such as aluminum orstainless steel. The chamber 10 may be frame grounded.

Above all, there will be explained a configuration of each componentwhich is not related to plasma generation in this inductively coupledplasma etching apparatus.

At a lower central region within the chamber 10, a circular plate-shapedsusceptor 12 may be provided horizontally. The susceptor 12 may mountthereon a target substrate such as a semiconductor wafer W and may serveas a high frequency electrode as well as a substrate holder. Thissusceptor 12 may be made of, for example, aluminum and may be supportedby a cylindrical insulating support 14 which may be extended uprightlyfrom a bottom of the chamber 10.

Between a cylindrical conductive support 16 which is extended uprightlyfrom a bottom of the chamber 10 along the periphery of the cylindricalinsulating support 14 and an inner wall of the chamber 10, an annularexhaust line 18 may be provided. Further, an annular baffle plate 20 maybe provided at an upper portion or an input of the exhaust line 18.Further, an exhaust port 22 may be provided at a bottom portion. Inorder for a gas flow within the chamber 10 to be uniformized withrespect to an axis of the semiconductor wafer W on the susceptor 12,multiple exhaust ports 22 equi-spaced from each other along acircumference may be provided. Each exhaust port 22 may be connected toan exhaust unit 26 via an exhaust pipe 24. The exhaust unit 26 mayinclude a vacuum pump such as a turbo molecular pump or the like. Thus,it may be possible to depressurize a plasma generation space within thechamber 10 to a required vacuum level. At an outside of a sidewall ofthe chamber 10, a gate valve 28 configured to open and close aloading/unloading port 27 of the semiconductor wafer W may be provided.

The susceptor 12 may be electrically connected to a high frequency powersupply 30 for RF bias via a matching unit 32 and a power supply rod 34.This high frequency power supply 30 may be configured to output avariable high frequency power RF_(L) having an appropriate frequency(typically, about 13.56 MHz or less) to control energies of ionsattracted into the semiconductor wafer W. The matching unit 32 mayaccommodate a variable reactance matching circuit for performingmatching between an impedance of the high frequency power supply 30 andan impedance of a load (mainly, susceptor, plasma and chamber). Thematching circuit may include a blocking capacitor configured to generatea self-bias.

An electrostatic chuck 36 for holding the semiconductor wafer W by anelectrostatic attraction force may be provided on an upper surface ofthe susceptor 12. Further, a focus ring 38 may be provided around theelectrostatic chuck 36 to annularly surround the periphery of thesemiconductor wafer W. The electrostatic chuck 36 may be formed byplacing an electrode 36 a made of a conductive film between a pair ofinsulating films 36 b and 36 c. A high voltage DC power supply 40 may beelectrically connected to the electrode 36 a via a switch 42 and acoated line 43. By applying a high DC voltage from the high voltage DCpower supply 40, the semiconductor wafer W can be attracted to and heldon the electrostatic chuck 36 by the electrostatic force.

A coolant cavity or a coolant path 44 of, e.g., a circular ring-shape,may be formed within the susceptor 12. A coolant, such as cooling watercw, having a certain temperature may be supplied into and circulatedthrough the coolant path 44 from a chiller unit (not illustrated) vialines 46 and 48. By adjusting the temperature of the cooling water cw,it may be possible to control a process temperature of the semiconductorwafer W held on the electrostatic chuck 36. Further, a heat transfergas, such as a He gas, may be supplied from a heat transfer gas supplyunit (not illustrated) into a space between an upper surface of theelectrostatic chuck 36 and a rear surface of the semiconductor wafer Wthrough a gas supply line 50. Furthermore, an elevating mechanism (notshown) including lift pins configured to move up and down verticallythrough the susceptor 12 may be provided to load and unload thesemiconductor wafer W.

Hereinafter, there will be explained a configuration of each componentwhich is related to plasma generation in this inductively coupled plasmaetching apparatus.

A ceiling or a ceiling plate of the chamber 10 may be separatedrelatively far from the susceptor 12. A circular dielectric window 52formed of, for example, a quartz plate may be airtightly provided as theceiling plate. Above the dielectric window 52, an antenna chamber 56 maybe provided as a part of the chamber 10. The antenna chamber 56 mayaccommodate therein a RF antenna 54 and shield this RF antenna 54 fromthe outside. Here, the RF antenna 54 may generate inductively coupledplasma within the chamber 10.

The RF antenna 54 is provided in parallel to the dielectric window 52.Desirably, the RF antenna 54 may be placed on the top surface of thedielectric window 52 and include an inner coil 58, an intermediate coil60, and an outer coil 62 with a certain gap therebetween in a radialdirection. The coils 58, 60, 62 are coaxially (preferably,concentrically) arranged. Further, the coils 58, 60, 62 are alsoarranged concentrically with each other as well as with the chamber 10or the susceptor 12.

In the illustrative embodiment, the term “coaxial” means that centralaxes of multiple objects having axisymmetric shape are aligned with eachother. As for multiple coils, respective coils surfaces may be offsetwith each other in an axial direction or may be aligned on the sameplane (positioned concentrically).

Further, the inner coil 58, the intermediate coil 60 and the outer coil62 are electrically connected in parallel between a high frequency powersupply line 68 from a high frequency power supply unit 66 for plasmageneration and a return line 70 toward a ground potential member (i.e.,between two nodes N_(A) and N_(B)). Here, the return line 70 as an earthline is grounded and is connected with a ground potential member (forexample, the chamber 10 or other member) that is electrically maintainedat a ground potential

A variable capacitor 86 is provided between the node N_(B) on the earthline 70 and the intermediate coil 60. Further, a variable capacitor 88is provided between the node N_(B) on the earth line 70 and the outercoil 62. Capacitances of these variable capacitors 86 and 88 may beindependently adjusted to a desired value within a certain range by acapacitance varying unit 90 under the control of a main controller 84.Hereinafter, a capacitor connected in series to the inner coil 58 willbe referred to as an “inner capacitor”; a capacitor connected in seriesto the intermediate coil 60 will be referred to as an “intermediatecoil”; and a capacitor connected in series to the outer coil 62 will bereferred to as an “outer capacitor.” All these capacitors are providedbetween the node N_(A) and N_(B).

The high frequency power supply unit 66 may include a high frequencypower supply 72 and a matching unit 74. The high frequency power supply72 is capable of outputting a variable high frequency power RF_(H)having a frequency (typically, equal to or higher than about 13.56 MHz)for generating plasma by an inductively coupled high frequency electricdischarge. The matching unit 74 has a reactance-variable matchingcircuit for performing matching between an impedance of the highfrequency power supply 72 and an impedance of load (mainly, RF antennaor plasma).

A processing gas supply unit for supplying a processing gas into thechamber 10 may include an annular manifold or buffer unit 76; multiplesidewall gas discharge holes 78; and a gas supply line 82. The bufferunit 76 may be provided at an inside (or outside) of the sidewall of thechamber 10 to be located at a position slightly lower than thedielectric window 52. The sidewall gas discharge holes 78 may be formedalong a circumference at a regular interval and opened to the plasmageneration space from the buffer unit 76. The gas supply line 82 may beextended from a processing gas supply source 80 to the buffer unit 76.The processing gas supply source 80 may include a flow rate controllerand an opening/closing valve (not shown).

The main controller 84 may include, for example, a micro computer andmay control an operation of each component within this plasma etchingapparatus, for example, the exhaust unit 26, the high frequency powersupplies 30 and 72, the matching units 32 and 74, the switch 42 for theelectrostatic chuck, the variable capacitors 86 and 88, the processinggas supply source 80, the chiller unit (not shown), and the heattransfer gas supply unit (not shown) as well as a whole operation(sequence) of the apparatus.

In order to perform an etching process in this inductively coupledplasma etching apparatus, when the gate valve 28 becomes open, thesemiconductor wafer W as a process target may be loaded into the chamber10 and mounted on the electrostatic chuck 36. Then, after closing thegate valve 28, an etching gas (generally, an mixture gas) may beintroduced into the chamber 10 from the processing gas supply source 80via the gas supply line 82, the buffer unit 76, and the sidewall gasdischarge holes 78 at a certain flow rate and a flow rate ratio.Subsequently, an internal pressure of the chamber 10 may be controlledto be a certain level by the exhaust unit 26. Further, the highfrequency power supply 72 of the high frequency power supply unit 66 isturned on, and the high frequency power RF_(H) for plasma generation isoutputted at a certain RF power level. A current of the high frequencypower RF_(H) is supplied to the inner coil 58, the intermediate coil 60and the outer coil 62 of the RF antenna 54 through the matching unit 74,the RF power supply line 68 and the return line 70. Meanwhile, the highfrequency power supply 30 may be turned on to output the high frequencypower RF_(L) for ion attraction control at a certain RF power level.This high frequency power RF_(L) may be applied to the susceptor 12 viathe matching unit 32 and the power supply rod 34. Further, a heattransfer gas (a He gas) may be supplied to a contact interface betweenthe electrostatic chuck 36 and the semiconductor wafer W from the heattransfer gas supply unit. Furthermore, the switch 42 is turned on, andthen, the heat transfer gas may be confined in the contact interface bythe electrostatic force of the electrostatic chuck 36.

Within the chamber 10, an etching gas discharged from sidewall gasdischarge holes 78 is diffused into processing space below thedielectric window 52. By the current of the high frequency power RF_(H)flowing in the coils 58, 60 and 62, magnetic force lines (magnetic flux)generated around these coils are transmitted to the processing space(plasma generation space) within the chamber 10 via the dielectricwindow 52. An induced electric field may be generated in an azimuthdirection within the processing space. Then, electrons accelerated bythis induced electric field in the azimuth direction may collide withmolecules or atoms of the etching gas so as to be ionized. In theprocess, donut-shaped plasma May be generated.

Radicals or ions in the donut-shaped plasma may be diffused in alldirections within the large processing space. To be specific, while theradicals are isotropically introduced and the ions are attracted by a DCbias, the radicals and the ions may be supplied on an upper surface(target surface) of the semiconductor wafer W. Accordingly, plasmaactive species may perform chemical and physical reactions on the targetsurface of the semiconductor wafer W so as to etch a target film into arequired pattern.

Herein, “donut-shaped plasma” is not limited to only ring-shaped plasmawhich is generated only at the radial outside in the chamber 10 withoutbeing generated at the radial inside (at a central area) therein.Further, “donut-shaped plasma” may include a state where a volume or adensity of the plasma generated at the radial outside is greater thanthat at the radial inside. Further, depending on a kind of a gas usedfor the processing gas, an internal pressure of the chamber 10, or thelike, the plasma may have other shapes instead of “a donut shape”.

In the inductively coupled plasma etching apparatus, the RF antenna 54is segmented into the inner coil 58, the intermediate coil 60, and theouter coil 62, which have different coil diameters. As a result, awavelength effect or an electric potential difference (voltage drop) inthe RF antenna 54 is effectively suppressed or reduced. Further, exceptfor the inner coil 58, the variable capacitors 86, 88 are connected inseries to the intermediate coil 60 and the outer coil 62, respectively.Therefore, plasma density distribution on the semiconductor wafer W canbe simply and effectively controlled.

[Basic Configuration and Operation of the RF Antenna]

FIGS. 2 and 3 illustrate a basic configuration of a layout and anelectric connection (circuit) of the RF antenna 54 in accordance withthe illustrative embodiment.

As illustrated in FIG. 2, the inner coil 58 includes a singlecircular-ring shaped coil with a gap or a space G_(i) therein, and theinner coil 58 has a constant radius. Further, the inner coil 58 ispositioned near a central portion of the processing chamber 10 in thediametrical direction. One end of the inner coil 58, i.e., an RF inputterminal 58in is connected to the RF power supply line 68 of the highfrequency power supply unit 66 via the first node N_(A) and theconnection conductor 92 extending upwardly. The other end of the innercoil 58, i.e., an RF output terminal 58out is connected to the earthline 70 via the second node N_(B) and the connection conductor 94extending upwardly.

The intermediate coil 60 includes a single circular-ring shaped coilwith a gap or a space G_(m) therein, and the intermediate coil 60 has aconstant radius. Further, the intermediate coil 60 is positioned at aportion of more outer than the inner coil 58 in the diametricaldirection in a middle portion of the processing chamber 10. One end ofthe intermediate coil 60, i.e., an RF input terminal 60in is adjacent tothe RF input terminal 58in of the inner coil 58 in the diametricaldirection. Further, the RF input terminal 60in is connected to the RFpower supply line 68 of the high frequency power supply unit 66 via thefirst node N_(A) and the connection conductor 96 extending upwardly. Theother end of the intermediate coil 60, i.e., an RF output terminal 60outis adjacent to the RF output terminal 58out of the inner coil 58 in thediametrical direction. Further, the RF output terminal 60out isconnected to the earth line 70 via the second node N_(B) and theconnection conductor 98 extending upwardly.

The outer coil 62 includes a single circular-ring shaped coil with a gapor a space G_(o) therein, and the outer coil 62 has a constant radius.The outer coil 62 is positioned at a portion of more outer than theintermediate coil 60 in the diametrical direction near the side wall ofthe processing chamber 10. One end of the outer coil 62, i.e., an RFinput terminal 62in is adjacent to the RF input terminal 60in of theintermediate coil 60 in the diametrical direction. The RF input terminal62in is connected to the RF power supply line 68 of the high frequencypower supply unit 66 via the first node N_(A) and the connectionconductor 100 extending upwardly. The other end of the outer coil 62,i.e., an RF output terminal 62out is adjacent to the RF output terminal60out of the intermediate coil 60 in the diametrical direction. The RFoutput terminal 62out is connected to the earth line 70 via the secondnode N_(B) and the connection conductor 102 extending upwardly.

As illustrated in FIG. 2, the connection conductors 92 to 102 upwardlyextending from the RF antenna 54 serve as branch lines or connectinglines in horizontal directions while spaced apart from the dielectricwindow 52 at a sufficiently large distance (i.e., at considerably highpositions). Accordingly, electromagnetic influence upon the coils 58, 60and 62 can be reduced.

In the above-described coil arrangement and segment connectionconfiguration within the RF antenna 54, when connecting from the highfrequency power supply 72 to the ground potential member via the RFpower supply line 68, the RF antenna 54, and the earth line 70, moredirectly, when connecting from the first node N_(A) to the second nodeN₈ via high frequency branch transmission lines of the coils 58, 60, 62within the RF antenna 54, the directions when passing through the innercoil 58, the intermediate coil 60, and the outer coil 62 are allcounterclockwise of FIG. 2 and identical to the circumferentialdirection.

In the inductively coupled plasma etching apparatus in accordance withthe illustrative embodiment, a high frequency current supplied from thehigh frequency power supply unit 66 flows through each of componentwithin the RF antenna 54. As a result, high frequency AC magnetic fieldsdistributed in loop shapes are formed around the inner coil 58, theintermediate coil 60 and the outer coil 62 of the RF antenna 54according to the Ampere's Law. Further, under the dielectric window 52,magnetic force lines passing through the processing space in the radialdirection are formed even in a relatively lower region.

In this case, a diametric directional (horizontal) component of amagnetic flux density in the processing space may be zero (0) constantlyat a central region and a periphery of the processing chamber 10regardless of a magnitude of the high frequency current. Further, theradial directional (horizontal) component of a magnetic flux density inthe processing space may have a maximum value at a certain portiontherebetween. A density distribution of the induced electric field inthe azimuth direction generated by the AC magnetic field of the highfrequency may have the same pattern as a magnetic flux densitydistribution in a diametrical direction. That is, an electron densitydistribution within the donut-shaped plasma in the diametrical directionmay substantially correspond to a current split within the RF antenna 54in a macro view.

The RF antenna 54 of the illustrative embodiment is different from atypical spiral coil wound from its center or inner peripheral end to anouter peripheral end thereof. That is, the RF antenna 54 includes thecircular ring-shaped inner coil 58 localized to the central portion ofthe antenna; the circular ring-shaped intermediate coil 60 localized tothe intermediate portion of the antenna; and the circular ring-shapedouter coil 62 localized to a peripheral portion of the antenna. Acurrent split in the RF antenna 54 may be concentrated in the vicinitiesof each of the coils 58, 60 and 62.

Here, a high frequency current I_(i) (hereinafter, referred to as an“inner coil current”) may be regular or uniform over the loop of theinner coil 58 and flows in the inner coil 58. A high frequency currentI_(m) (hereinafter, referred to as an “intermediate coil current”) maybe regular or uniform over the loop of the intermediate coil 60 andflows in the intermediate coil 60. A high frequency current I_(o)(hereinafter, referred to as an “outer coil current”) may be regular oruniform over the loop of the outer coil 62 and flows in the outer coil62.

Therefore, in the donut-shaped plasma generated below (inside) thedielectric window 52 of the processing chamber 10, as shown in FIG. 3),a current density (i.e. plasma density) may be remarkably increased(maximized) at positions right below the inner coil 58, the intermediatecoil 60 and the outer coil 62. Thus, a current density distributionwithin the donut-shaped plasma may not be uniform in a diametricaldirection and may have an uneven profile. However, since the plasma isdiffused in all directions within the processing space of the processingchamber 10, a plasma density in a vicinity of the susceptor 12, i.e. onthe substrate W, may become very uniform.

In the present illustrative embodiment, the inner coil 58, theintermediate coil 60 and the outer coil 62 have the circular ringshapes. Further, since a regular or uniform high frequency currents flowin the circumferential directions of the coils, a plasma densitydistribution can constantly be uniformized in the circumferentialdirections of the coils in the vicinity of the susceptor 12, i.e., onthe substrate W as well as within the donut-shaped plasma.

Further, in the radial direction, by varying and setting theelectrostatic capacitances C₈₆ and C₈₈ of the intermediate capacitor 86and the outer capacitor 88 to have appropriate values within certainranges, it is possible to adjust a balance between the currents I_(i),I_(m) and I_(o) flowing in the inner coil 58, the intermediate coil 60and the outer coil 62, respectively. Accordingly, plasma densitydistribution within the donut-shaped plasma can be controlled asdesired. Thus, plasma density distribution in the vicinity of thesusceptor 12, i.e., on the substrate W can be controlled as desired, andplasma density distribution can be easily uniformized with highaccuracy.

In the illustrative embodiment, the wavelength effect and the voltagedrop within the RF antenna 54 depend on a length of each of the coils58, 60, 62. Accordingly, by setting the length of each of the coils toprevent the wavelength effect from occurring in the coils 58, 60, 62,both the wavelength effect and the voltage drop within the RF antenna 54can be reduced. In order to prevent the wavelength effect, the length ofeach of the coils 58, 60, 62 is desirably shorter than a ¼ wavelength ofthe high frequency RF_(H).

The condition that the length of each the coil is less than a ¼wavelength of the high frequency RF_(H) is easily satisfied when adiameter of a coil is small, and the number of windings is small.Accordingly, in the RF antenna, the inner coil 58 having a smallestdiameter can be easily subject to a configuration of a multiple numberof windings. The outer coil 62 having a largest diameter is desirablysubject to a single winding, rather than a multiple number of windings.The intermediate coil 60 depends on a diameter of the semiconductorwafer W, the frequency of the high frequency RF_(H), and the like.However, the intermediate coil 60 is desirably subject to a singlewinding, like the outer coil 62.

[Functions of Capacitors Added to the RF Antenna]

The core technical feature of the illustrative embodiment lies in thatthe RF antenna 54 is segmented in parallel in the diametrical directioninto the three coils 58, 60, 62 having different coil diameters.Further, the variable intermediate capacitor 86 and the variable outercapacitor 88 are electrically connected in series to the intermediatecoil 60 and the outer coil 62, respectively. Meanwhile, any reactancedevice (in particular, a capacitor) is not connected to the inner coil58.

Here, as illustrated in FIG. 4, it is assumed that no capacitor isconnected to the RF antenna. In such case, more remarkable and strongerplasma than that generated directly under the intermediate coil 60 andthe outer coil is generated directly under the inner coil 58. The reasonthat the remarkable and strong plasma is generated directly under theinner coil 58 will be described. A self-inductance L of a singlecircular ring-shaped coil is expressed by the formula (1) below, where athickness (radius) of a coil conducting wire is represented as “a”, anda coil diameter (radius) is represented as “r”.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} (1)} \right\rbrack & \; \\{L = {r\; {\mu_{0}\left( {{\log \frac{8\; r}{a}} - 2} \right)}}} & (1)\end{matrix}$

Here, μ_(o) is a vacuum permeability, and the coil diameter (radius) ris a middle value between a radius of an inner periphery of a coil and aradius of an outer periphery of a coil.

From the formula (1), the self-inductance L is linearly proportional tothe coil radius r. When a frequency of the high frequency is f, animpedance Z of the circular ring-shaped coil is 2πfL (Z=2πfL), andproportional to the self-inductance L. Accordingly, if the coil radiusesof the inner coil 58, the intermediate coil 60, and the outer coil 62are, for example, about 50 mm, about 100 mm, and about 150 mm,respectively, the inner coil current I_(i) flows in the inner coil 58,and an amount of the inner coil current I_(i) is approximately 2 timeslarger than the intermediate coil current I_(m) flowing in theintermediate coil 60, and approximately 3 times larger than the outercoil current I_(o) flowing in the outer coil 62. Density of the plasmagenerated by the circular ring-shaped coils is slightly lowered inefficiency when the radiuses of the coils are small. However, the plasmadensity generally depends on an amount of the coil currents, regardlessof the radiuses of the coils. As such, several times stronger plasmathan that generated directly under the intermediate coil 60 and theouter coil 62 is generated directly under the inner coil 58.Accordingly, the plasma density distribution near the susceptor 12,i.e., on the semiconductor wafer W has a profile, where the centralportion in the diametrical direction is protruded and becomes high.

However, a coil has a positive reactance, and a capacitor has a negativereactance. When a capacitor is connected to a coil, thenegative-reactance of the capacitor removes the positive reactance ofthe coil. As a result, a combined reactance becomes lower than thereactance of the coil. Accordingly, increasing the amount of the innercoil current I_(i) by connecting a capacitor to the inner coil 58 doesnot have an effect on the uniformity of the plasma density distribution,and rather, results in a reverse effect thereon.

This is the same when the coils 58, 60, 62 are configured by spiralcoils having N_(i), N_(m), and N_(o) turns, respectively. That is, sincean inductance of each coil is proportional to the number of turns (thenumber of windings), an impedance of each coil is also proportional tothe number of turns. Accordingly, a ratio of the coil currents (I_(i),I_(o)) flowing in the inner coil 58 and the outer coil 62, respectively,is I_(i):I_(o)=r_(o)*N_(o)*N_(i): r_(i)*N_(i)*N_(o)=r_(o): r_(i).Meanwhile, the plasma density (n_(i), n_(o)) generated directly undereach of the coils 58 and 62 is determined by multiplying the coilcurrent and the number of turns. Accordingly, a ratio of the plasmadensities (n_(i), n_(o)) generated directly under the inner coil 58 andthe outer coil 62 is n_(i): n_(o)=r_(o)*N_(o)*N_(i):r_(i)*N_(i)*N_(o)=r_(o):r_(i), which depends on a ratio of radiuses. Theratio of the coil currents and the plasma densities is identicallyapplied to the case of the inner coil 58 and the intermediate coil 60.As described, constantly, stronger plasma than those generated directlyunder the intermediate coil 60 and the outer coil 62 is generateddirectly under the inner coil 58.

Actually, an impedance of a wiring from the output terminal of thematching unit 74 to the corresponding coil cannot be ignored. A lengthof the wiring is determined by a height from the output terminal of thematching unit 74 to the RF antenna 54. Thus, it may be assumed that thelength of the wiring is the same among the inner side, the intermediateside, and the outer side. Further, it is assumed that the wiringimpedance is the same at the inner side, the intermediate side, and theouter side. In such case, if the outer coil 62 and the inner coil 58have impedances of about 75Ω and about 25Ω, respectively, when thewiring impedance is approximately about 10Ω,I_(i):I_(o)=(75+10):(25+10)=85:35=2.41:1. Thus, there is still adifference two or more times between the impedances of the coils and thewiring impedance.

When a pressure is relatively high (generally, more than about 100mTorr), and thus, plasma is difficult to be diffused, theabove-described balance is achieved at the inner side and the outerside. However, when a pressure is lowered, and thus, the plasma is easyto be diffused, a plasma density at a central portion is furtherprotruded and becomes high.

Accordingly, there is established a theory providing that under anyconditions, a plasma density directly under an innermost coil, among amultiple number of coils electrically connected in parallel to oneanother and having different diameters, becomes relatively high.

In the illustrative embodiment, based on the above-described theory, nocapacitor is added (connected) to the inner coil 58 between the bothterminals (between the first node N_(A) and the second node N_(B)) ofthe RF antenna 54. The variable capacitors 86, 88 are connected inseries to the intermediate coil 60′ and the outer coil 62, respectively.By adjusting the electrostatic capacitances C₈₆, C₈₈ of the variablecapacitors 86, 88 in consideration of reducing a combined reactance, theamounts of the coil currents I_(m), I_(o) flowing in the intermediatecoil 60 and the outer coil 62, respectively, are properly increased. Inthis manner, it is possible to conform the coil currents I_(i), I_(m),I_(o) to be in substantially the same amounts. Alternatively, it ispossible to make the intermediate coil current I, and/or the outer coilcurrent I_(o) larger than the inner coil current I_(i). Here, theincreasing amounts of the coil currents I_(m), I_(o) by the variablecapacitors 86, 88 flow in the intermediate coil and the outer coil 62,and contribute to the plasma generation. Thus, no waste of highfrequency power is generated.

In general, in order to correct the profile, where a plasma density isprotruded and becomes high at the central portion of the diametricaldirection, it is effective to adjust a ratio (balance) of the inner coilcurrent I_(i) flowing in the inner coil 58 and the outer coil currentI_(o) flowing in the outer coil 62. In the illustrative embodiment, aratio of the coil currents I_(i), I_(o) can be adjusted simply byvarying the electrostatic capacitance C₈₈ of the outer capacitor 88.

In this case, in order to uniformize the plasma density distributionunder various conditions, it is desirable to set a variable range ofmultiplication (I_(o)*n_(o)) of the outer coil current I_(o) and thenumber of turns n_(o) the outer coil 62 to have a lower limit valuesmaller than and an upper limit value larger than multiplication(I_(i)*n_(i)) of the inner coil current I_(i) and the number of turnsn_(i) of the inner coil 58. The ratio of the inner coil current I_(i)and the outer coil current I_(o) is proportional to a ratio ofreciprocal numbers of an impedance of the inner coil 58 (hereinafter,referred to as an “inner impedance”) and a combined impedance of theouter coil 62 and the outer capacitor 88 (hereinafter, referred to as an“outer combined impedance”). Accordingly, if the inner impedance (fixedvalue) is represented as Z_(i), and minimum and maximum values for theouter combined impedance (variable value) are represented asZ_(o)(_(min)) and Z_(o)(_(max)), respectively, the above-describedcondition for a multiplication of the coil current and the number ofturns is expressed as follows:

[Formula (2)]

|N _(o) /Z _(o(max)) |<|N _(i) /Z _(i) |<|N _(o) /Z _(o(min))|  (2)

Further, the inner impedance Z_(i) and the outer combined impedanceZ_(o) depend on their respective average coil radiuses, except for theelectric connecting portion. An effect of the electric connectingportion cannot be ignored, but is not dominant. Thus, theabove-described condition may be expressed as follows:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} (3)} \right\rbrack & \; \\{\frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\max)}}}} < \frac{1}{2\pi \; {fr}_{i}{\mu_{0}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} < \frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\min)}}}}} & (3)\end{matrix}$

Here, C_(o)(_(min)) is a value of the electrostatic capacitance C₈₈ ofthe outer capacitor 88 when the outer combined impedance Z_(o) isminimum within an adjustable range. C_(o)(_(max)) is a value of theelectrostatic capacitance C₈₈ of the outer capacitor 88 when the outercombined impedance Z_(o) is maximum. C_(o)(_(min)) and C_(o)(_(max)) maynot be the same as the minimum value and the maximum value of thevariable range. The outer combined impedance Z_(o) becomes minimum, whenthe outer coil 62 and the outer capacitor 88 cause a series resonance.C_(o)(_(min)) is a value of the electrostatic capacitance C₈₈ of theouter capacitor 88 at that time. The outer combined impedance Z_(o)becomes maximum, when the electrostatic capacitance C₈₈ of the outercapacitor 88 becomes apart from the series resonance point up to anupper or lower limit of the variable range. C_(o)(_(max)) is a value ofthe electrostatic capacitance C₈₈ of the outer capacitor 88 at thattime.

As described, no capacitor is connected to the inner coil 58, and theouter capacitor 88 is connected in series to the outer coil 62. Byvarying the electrostatic capacitance C₈₈ of the outer capacitor 88, theratio of the inner coil current I_(i) and the outer coil current I_(o)is properly adjusted. Also, a general profile of the plasma densitydistribution within the donut-shaped plasma generated directly under theRF antenna 54 (in particular, balance between the central portion andthe peripheral portion) can be properly controlled.

In the illustrative embodiment, the intermediate coil 60 is arrangedbetween the inner coil 58 and the outer coil 62. The variableintermediate capacitor 86 is connected in series to the intermediatecoil 60. This configuration is intended to minutely control the plasmadensity distribution (in particular, at the intermediate portion) withinthe donut-shaped plasma. The configuration is useful when generatingplasma under a low pressure or generating plasma of a large diameter.

Instead of providing the intermediate coil 60, it is considered to formthe outer coil 62 in a spiral shape to cover the area of theintermediate portion of the antenna 54 with the outer coil 62. In thiscase, however, the same coil current I_(o) flows in all sections of theouter coil 62. Thus, the plasma density directly under the intermediateportion of the antenna may become relatively high. It becomes difficultto achieve a uniform profile, for example, in the diametrical direction.

In actual processes, there is a case where a desirable profile (e.g.,flat) in the whole diametrical direction is obtained by forciblyreducing the plasma density within the donut-shaped plasma directlyunder the intermediate portion of the antenna. In particular, thissituation easily occurs when an amount of the outer coil current I_(o)is rapidly increased in order to raise peripheral plasma density to adesired level.

In this case, the intermediate capacitor 86 is used effectively. Thatis, the electrostatic capacitance C₈₆ of the intermediate capacitor 86is varied in a range lower than a value of a series resonance point. Asa result, a combined reactance of the intermediate coil 60 and theintermediate capacitor 86 (hereinafter, referred to as an “intermediatecombined reactance”) becomes a negative value. Accordingly, theintermediate coil current I_(m) flows in a direction opposite to thecircumferential direction and in a certain current amount (inparticular, it is also possible to slightly increase the current amountfrom the state of substantially zero (0)). Accordingly, the plasmadensity within the donut-shaped plasma directly under the intermediatecoil can be locally and easily controlled. Furthermore, the plasmadensity distribution in the whole diametrical direction near thesusceptor 12, i.e., on the semiconductor wafer W can be easilycontrolled.

The function of the intermediate capacitor 86 has been verified by theexperiment shown in FIGS. 5A and 5B. In the experiment, as illustratedin FIG. 5A, the inner coil 58 of the RF antenna 54 is formed with twowindings (2 turns) having a diameter of about 100 mm. The intermediatecoil 60 and the outer coil 62 are formed with a single winding (singleturn) having diameters of about 200 mm and about 300 mm, respectively.As primary process conditions, the frequency of the high frequencyRF_(H) is about 13.56 MHz, the RF power is about 1500 W, a pressure inthe processing chamber 10 is about 100 mTorr, the processing gas is amixture gas of Ar and O₂, and a flow rate of the processing gas isAr/O₂=about 300/30 sccm.

In the experiment, the electrostatic capacitance C₈₈ of the outercapacitor 88 is fixed at about 560 pF. The electrostatic capacitance C₈₆of the intermediate capacitor 86 is varied to about 13 pF, to about 40pF, and to about 64 pF. In this case, it is confirmed that as shown inFIG. 5B, the intermediate coil current I_(m) is changed to about −0.4 A,to about −5.0 A, and to about −11.2 A, and the electron density N_(e)(i.e., plasma density) near a portion directly under the intermediatecoil 60 can be locally and properly lowered. When C₈₆ is about 13 pF,the inner and outer coil currents I_(i), I_(o) are about 16.4 A andabout 18.3 A, respectively. When C₈₆ is about 40 pF, the inner and outercoil currents I_(i), I_(m) are about 17.4 A and about 19.4 A,respectively. When C₈₆ is about 64 pF, the inner and outer coil currentsI_(i), I_(o) are about 19.0 A and about 20.1 A, respectively. If anamount of the intermediate coil current I_(m) flowing in the oppositedirection increases, amounts of the inner coil current I_(i) and theouter coil current I_(o), which flow in the forward direction, slightlyincrease. However, a ratio (balance) of the coil currents I_(i), I_(o)usually does not vary.

With respect to other functions in the RF antenna 54 of the presentillustrative embodiment, it is possible to vary the electrostaticcapacitance C₈₈ of the outer capacitor 88 in a range lower than thevalue of the series resonance point. As a result, a combined reactanceof the outer coil 62 and the outer capacitor 88 (hereinafter, referredto as an “outer combined reactance”) becomes a negative value.Accordingly, it is possible to make the outer coil current I_(o) flow inthe opposite direction. For example, if plasma diffuses excessivelytoward the outer side of the diametrical direction within the processingchamber 10, the inner wall of the processing chamber 10 may be easilydamaged. In such case, it is possible to make the coil current I_(o)flow in a direction opposite to the outer coil 62, thereby confining theplasma in the inner side of the outer coil 62. As a result, the damageof the inner wall in the processing chamber 10 can be prevented. Thisfunction is effective, for example, when a multiple number ofintermediate coils 60 having different coil diameters are arranged whilebeing spaced from one another in the diametrical direction, or when theouter coil 62 is arranged at a further outer side of the diametricaldirection than the susceptor 12.

In the illustrative embodiment, by making at least one of theelectrostatic capacitances C₈₆, C₈₈ of the intermediate capacitor 86 andthe outer capacitor 88 close to a value when a series resonance isgenerated, it is possible to reduce the inner coil current I_(i) flowingin the inner coil 58. By making at least one of the electrostaticcapacitances C₈₆, C₈₈ of the intermediate capacitor 86 and the outercapacitor 88 apart from the value when the series resonance isgenerated, it is possible to increase the inner coil current I_(i). Thatis, the inner coil current I_(i), the intermediate coil current I_(m),and the outer coil current I_(o) are in a ratio ofI_(i):I_(m):I_(o)=(1/Z_(i)):(1/Z_(m)):(1/Z_(o)). Accordingly, as C₈₆and/or C₈₈ are close to the value when the series resonance isgenerated, Z_(m) and/or Z_(o) become smaller values. Here, I_(m) and/orI_(o) are relatively increased, and I_(i) becomes smaller. Meanwhile, asC₈₆ and/or C₈₈ are apart from the value when the series resonance isgenerated, Z_(m) and/or Z_(o) become large values. Here, I_(m) and/orI_(o) are relatively decreased, and I_(i) becomes larger.

[Another Testing Example or a Modified Example for the RF Antenna]

In the illustrative embodiment, in controlling (in particular,uniformizing) the plasma density distribution, no capacitor is connectedto the inner coil 58, considering that an amount of the inner coilcurrent I_(i) does not need to be varied in order to increase thecurrent amount thereof.

However, there is a case where positively or forcibly controlling anamount of the inner coil current I_(i) is effective. For example, when apressure in the processing chamber 10 is low, the plasma tends to begathered at a central portion of the diametrical direction. As describedabove, this problem may be resolved by varying the electrostaticcapacitance C₈₈ of the outer capacitor 88 and adjusting a ratio of theinner coil current I_(i) and the outer coil current I_(o). However, theproblem may not be completely resolved.

As illustrated in FIGS. 6A and 6B, the fixed or semi-fixed innercapacitor 104 is electrically connected in series to the inner coil 58between the first node N_(A) and the second node N_(B) in the RF antenna54. In this configuration, it is also possible to select or adjust theelectrostatic capacitance C₁₀₄ of the inner capacitor 104 to be adesired value. As a result, a combined reactance X_(i) of the inner coil58 and the inner capacitor 104 (hereinafter, referred to as an “innercombined reactance”) becomes a desired value. When the fixed orsemi-fixed capacitor 104 and the variable outer capacitor 88 areprovided, the condition formulas (2) and (3) are also applied withrespect to the number of turns of each of the coils or the variablecontrol of the electrostatic capacitance C₈₈ of the outer capacitor 88.

Although a freedom degree or an accuracy of the plasma densitydistribution control is somewhat decreased, in order to reduce costs, avariable capacitor for the inner capacitor 104 and a fixed or semi-fixedcapacitor for the outer capacitor 88 may be used, as illustrated inFIGS. 7A and 7B. Although not illustrated herein, a fixed or semi-fixedcapacitor for the intermediate capacitor 86 may be used. Further, theintermediate capacitor 86 may be omitted.

When the fixed or semi-fixed outer capacitor 88 and the variable innercapacitor 104 are provided, in uniformizing the plasma densitydistribution under various conditions, it is desirable to set a variablerange of multiplication (I_(i)*n_(i)) of the inner coil current I_(i)and the number of turns n_(i) of the inner coil 58 to have a lower limitvalue smaller and an upper limit value larger than multiplication(I_(o)*n_(o)) of the outer coil current I_(o) and the number of turns(n_(o)) of the outer coil 62. The ratio of the inner coil current I_(i)and the outer coil current I_(o) is proportional to a ratio ofreciprocal numbers of the combined impedance (inner combined impedance)of the inner coil 58 and the inner capacitor 104, and the combinedimpedance (outer combined impedance) of the outer coil 62 and the outercapacitor 88. Accordingly, if the outer impedance (fixed value) isrepresented as Z_(o), and minimum and maximum values of the innercombined impedance (variable value) are represented as Z_(i)(_(min)) andZ_(i)(_(max)), respectively, the above-described condition for themultiplication of the coil current and the number of turns is expressedas follows:

[Formula (4)]

|N _(i) /Z _(i(max)) |<|N ₀ /Z _(o) |<|N _(i) /Z _(i(min))|  (4)

The inner combined impedance Z_(i) and the outer combined impedanceZ_(o) depend on their respective average coil radiuses, except for theelectric connecting portion. An effect of the electric connectingportion cannot be ignored, but is not dominant. Thus, theabove-described condition may be expressed as follows:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} (5)} \right\rbrack & \; \\{\frac{1}{{2\pi \; {fr}_{i}{\mu_{0}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{i}C_{i{(\max)}}}} < \frac{1}{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} < \frac{1}{{2\pi \; {fr}_{i}{\mu_{0}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{i}C_{i{(\min)}}}}} & (5)\end{matrix}$

Here, C_(i)(_(min)) is a value of the electrostatic capacitance C₁₀₄ ofthe inner capacitor 104 when the inner combined impedance Z_(i) isminimum within an adjustable range. C_(i)(_(max)) is a value of theelectrostatic capacitance C₁₀₄ of the inner capacitor 104 when the innercombined impedance Z_(i) is maximum. C_(i)(_(min)) and C_(i)(_(max)) maynot be the same as a minimum value and a maximum value of the variablerange of C₁₀₄. The inner combined impedance Z_(i) becomes minimum, whenthe inner coil 58 and the inner capacitor 104 cause a series resonance.C_(i)(_(min)) is a value of the electrostatic capacitance C₁₀₄ of theinner capacitor 104 at that time. Further, the inner combined impedanceZ_(i) becomes maximum, when the electrostatic capacitance C_(DM) of theinner capacitor 104 becomes apart from the series resonance point up toan upper or lower limit of the variable range. C_(i)(_(max)) is a valueof the electrostatic capacitance C₁₀₄ of the inner capacitor 104 at thattime.

The variable inner capacitor 104 is connected to the inner coil 58, anda fixed or semi-fixed outer capacitor 88 is connected in series to theouter coil 62. By varying the electrostatic capacitance C₁₀₄ of theinner capacitor 104, the ratio of the inner coil current I_(i) and theouter coil current I_(o) is properly adjusted. Also, it is possible toproperly control a profile of the plasma density distribution within thedonut-shaped plasma generated directly under the RF antenna 54 (inparticular, a balance of the central portion and the peripheralportion).

In accordance with a modified example, in order to positively orforcibly reduce the amount of the inner coil current I_(i), a variableinductor 106 may be electrically connected in series to the inner coil58 between the first node N_(A) and the second node N_(B). Also, it ispossible to substitute the intermediate capacitor 86 with anothervariable inductor, or it is possible to omit the outer capacitor 88 forreducing costs.

The loop shape of each of the coils 58, 60, 62 in the RF antenna 54 isnot limited to a circular shape. For example, as illustrated in FIG. 9,the loop shape of each of the coils 58, 60, 62 may be square accordingto the shape of the target object to be processed. When the loop shapeof each of the coils 58, 60, 62 is polygonal, the variable intermediatecapacitor 86 and the outer capacitor 88 may be electrically connected inseries to the intermediate coil 60 and the outer coil 62, respectively,and no capacitor may be connected to the inner coil 58.

An inductance L of a rectangular coil, where lengths of two sides arerepresented as “a” and “b”, and a thickness of the coil is representedas a radius “d”, is expressed by the formula (6) below.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} (6)} \right\rbrack & \; \\{L = {\frac{\mu_{0}}{\pi}\left\{ {{{- a}\; {\log \left( {a + \sqrt{a^{2} + b^{2}}} \right)}} - {b\; {\log \left( {b + \sqrt{a^{2} + b^{2}}} \right)}} + {\left( {a + b} \right)\log \frac{2\; {ab}}{d}} + {2\sqrt{a^{2} + b^{2}}} - {\frac{7}{4}\left( {a + b} \right)}} \right\}}} & (6)\end{matrix}$

As shown from the formula (6), the inductance L of the rectangular coilis substantially proportional to the lengths of the two sides “a”, “b”.Accordingly, even when no capacitor is connected to the inner coil 58 asthe same as in the circular coil, the plasma density distribution can besimply or easily controlled by the intermediate capacitor 86 and theouter capacitor 88.

FIG. 10 shows an example where the coils (inner coil 58/intermediatecoil 60/outer coil 62) of the RF antenna 54 are formed with a pair ofspiral coils. The coils are arranged in a spatially and electricallyparallel. The spiral coils may be used unless the wavelength effect isreally a problem.

In the illustrated configuration example, the inner coil 58 is formedwith a pair of spiral coils 58 a, 58 b having a multiple number of turns(respectively, 2 turns in the illustrated example) and being deviated180° from the circumferential direction. The spiral coils 58 a, 58 b areelectrically connected in parallel between a node N_(c) provided at adownstream side lower than the node N_(A) of the high frequency powersupply 72 and a node N_(D) provided at an upstream side higher than thenode N_(B) of the earth line 70.

The intermediate coil 60 is formed with a pair of spiral coils 60 a, 60b each having a multiple number of turns (respectively, 2 turns in theillustrated example) and being deviated 180° from the circumferentialdirection. The spiral coils 60 a, 60 b are electrically connected inparallel between a node N_(E) provided at a downstream side lower thanthe node N_(A) of the high frequency power supply 72 and a node N_(F)provided at an upstream side higher than the node N_(B) (and theintermediate capacitor 86) of the earth line 70.

The outer coil 62 is formed with a pair of spiral coils 62 a, 62 b eachhaving a multiple number of turns (respectively, 2 turns in theillustrated example) and being deviated 180° from the circumferentialdirection. The spiral coils 62 a, 62 b are electrically connected inparallel between a node N_(G) provided at a downstream side lower thanthe node N_(A) of the high frequency power supply 72 and a node N_(H)provided at an upstream side higher than the node N_(B) (and the outercapacitor 88) of the earth line 70.

When the inner coil 58, the intermediate coil 60, and the outer coil 62are segmented into k_(i), k_(m), and k_(o) spiral coils in parallel,respectively, if no capacitor is connected to the RF antenna 54, a ratioof plasma densities n_(i), n_(m), n_(o), which is generated directlyunder the coils 58, 60, 62, respectively, is expressed by anapproximation equation (3) below.

n _(i) :n _(m) :n _(o)≈(k _(i) /ri):(k _(m) /ri):(k _(o) /ri)  (3)

As in the illustrated example, if the number of segments k_(i), k_(m),k_(o) is the same (2 segments), the plasma density generated directlyunder the inner coil 58, i.e., the plasma in the central portion becomesrelatively higher. In this case, the most desirable configuration isthat no capacitor is connected to the inner coil 58, and the variableintermediate capacitor 86 and the outer capacitor 88 are connected inseries to the intermediate coil 60 and the outer coil 62, respectively.

With respect to another modified example, as illustrated in FIG. 11, itis possible to connect an intermediate capacitor 86 and an outercapacitor 88 between the first node N_(A) of the high frequency powersupply 72 and the RF inlet terminals 60in, 62in of the intermediate coil60 and the outer coil 62. Connecting a capacitor or an inductor foradjusting impedance between the first node N_(A) and each of the coilsmay be applied to another experimental example or another modifiedexample (FIGS. 6A, 8, 9, and 10).

FIG. 12 illustrates the configuration where an output common capacitor108 is connected between the second node N_(B) and the earth line 70 (oron the earth line 70) at the terminal end of the RF antenna 54, andelectrically connected in series to all the coils 58, 60, 62 of the RFantenna 54. The output terminal (terminal end) common capacitor 108 maybe a common fixed capacitor or a variable capacitor.

The output terminal (terminal end) common capacitor 108 has a functionof adjusting the whole impedance of the RF antenna 54, and a function ofsuppressing ion sputter performed on the ceiling plate or the dielectricwindow 52 by serially raising the whole potential of the RF antenna 54from a ground potential.

Although not illustrated herein, it is possible to provide other coilsat the diametrical direction inner side of the inner coil 58 and/or thediametrical direction outer side of the outer coil 62 in the RF antenna54. As a result, total 4 or more coils are arranged in the diametricaldirection at intervals while being electrically connected in parallel toone another.

In the above-described embodiment, the illustrated configuration of theinductively coupled plasma etching apparatus is nothing more than anexample. Not only each component of the plasma generating mechanism butalso each component which is not directly relevant to plasma generationcan be modified in various manners.

By way of example, the basic shape of the RF antenna may be a dome shapebesides the planar shape mentioned above. Further, it may be alsopossible to have configuration in which a processing gas is introducedinto the chamber 10 from the processing gas supply unit through aceiling. Furthermore, it may be also possible not to apply a highfrequency power RF_(L) for DC bias control to the susceptor 12.

The inductively coupled plasma processing apparatus or the inductivelycoupled plasma processing method of the present disclosure can beapplied to, not limited to a plasma etching technology, other plasmaprocesses such as plasma CVD, plasma oxidation, plasma nitridation, andsputtering. Further, the target substrate in the present disclosure mayinclude, but is not limited to a semiconductor wafer, various kinds ofsubstrates for a flat panel display or photo mask, a CD substrate, and aprint substrate.

1. A plasma processing apparatus, comprising: a processing chamberhaving a dielectric window; a substrate holding unit for holding thereona processing target substrate within the processing chamber; aprocessing gas supply unit configured to supply a processing gas intothe processing chamber in order to perform a plasma process on theprocessing target substrate; an RF antenna provided outside thedielectric window in order to generate plasma of the processing gaswithin the processing chamber by inductive coupling; and a highfrequency power supply unit configured to supply a high frequency powerhaving a frequency for generating a high frequency electric discharge ofthe processing gas in the RF antenna, wherein the RF antenna includes aninner coil, an intermediate coil, and an outer coil with gapstherebetween in a radial direction, respectively, and the inner coil,the intermediate coil and the outer coil are electrically connected toone another in parallel between a first node and a second node providedin high frequency transmission lines of the high frequency power supplyunit, a variable intermediate capacitor and a variable outer capacitorare provided between the first node and the second node, and thevariable intermediate capacitor is electrically connected in series tothe intermediate coil, and the variable outer capacitor is electricallyconnected in series to the outer coil, and no reactance device isconnected to the inner coil between the first node and the second node.2. The plasma processing apparatus of claim 1, wherein if the number ofwindings of the inner coil and the number of windings of the outer coilare N_(i) and N_(o), respectively, an impedance of the inner coil isZ_(i), and a maximum value and a minimum value of a combined impedanceof the outer coil and the outer capacitor obtained by varying anelectrostatic capacitance of the outer capacitor are Z_(o)(_(max)) andZ_(o)(_(min)), respectively, the following inequation is established:|N _(o) /Z _(o(max)) |<|N _(i) /Z _(i) |<|N _(o) /Z _(o(min))|
 3. Theplasma processing apparatus of claim 2, wherein each of the inner coiland the outer coil has a circular ring shape, and if radiuses of theinner coil and the outer coil are represented as “r_(i)” and “r_(o)”,respectively, a radius of a wire thickness of each of the coils isrepresented as “a”, a vacuum permeability is represented as “μ_(o)”, andwhen the combined impedance of the outer coil and the outer capacitorbecomes the maximum value Z_(o)(_(max)) and the minimum valueZ_(o)(_(min)), an electrostatic capacitance of the outer capacitor isrepresented as “C_(o)(_(max))” and “C_(o)(_(min))”, respectively, thefollowing inequation is established:$\frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\max)}}}} < \frac{1}{2\pi \; {fr}_{i}{\mu_{0}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} < \frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\min)}}}}$4. The plasma processing apparatus of claim 1, wherein directions ofcurrents flowing in the inner coil, the intermediate coil, and the outercoil are identical to one another in a circumferential direction.
 5. Theplasma processing apparatus of claim 1, wherein a direction of thecurrent flowing in the intermediate coil is opposite to a direction ofthe current flowing in the inner coil in the circumferential direction.6. The plasma processing apparatus of claim 5, wherein the intermediatecapacitor has a value of electrostatic capacitance smaller than a valuewhen the intermediate capacitor and the intermediate coil generate aseries resonance.
 7. The plasma processing apparatus of claim 1, whereina direction of the current flowing in the outer coil is identical to adirection of the current flowing in the inner coil in thecircumferential direction.
 8. The plasma processing apparatus of claim7, wherein the outer capacitor has a value of electrostatic capacitancelarger than a value when the outer capacitor and the outer coil generatea series resonance.
 9. The plasma processing apparatus of claim 1,wherein a direction of the current flowing in the outer coil is oppositeto a direction of a current flowing in the inner coil in thecircumferential direction.
 10. The plasma processing apparatus of claim9, wherein the outer capacitor has a value of electrostatic capacitancesmaller than a value when the outer capacitor and the outer coilgenerate a series resonance.
 11. A plasma processing apparatus,comprising: a processing chamber having a dielectric window; a substrateholding unit for holding thereon a processing target substrate withinthe processing chamber; a processing gas supply unit configured tosupply a processing gas into the processing chamber in order to performa plasma process on the processing target substrate; an RF antennaprovided outside the dielectric window in order to generate plasma ofthe processing gas within the processing chamber by inductive coupling;and a high frequency power supply unit configured to supply a highfrequency power having a frequency for generating a high frequencyelectric discharge of the processing gas in the RF antenna, wherein theRF antenna includes an inner coil, an intermediate coil, and an outercoil with gaps therebetween in a radial direction, respectively, and theinner coil, the intermediate coil and the outer coil are electricallyconnected to one another in parallel between a first node and a secondnode provided in high frequency transmission lines of the high frequencypower supply unit, and a fixed or semi-fixed inner capacitor, a variableintermediate capacitor, and a variable outer capacitor are providedbetween the first node and the second node, and the fixed or semi-fixedinner capacitor is electrically connected to the inner coil, thevariable intermediate capacitor is electrically connected in series tothe intermediate coil, and the variable outer capacitor is electricallyconnected in series to the outer coil.
 12. The plasma processingapparatus of claim 11, wherein if the number of windings of the innercoil and the number of windings of the outer coil are N_(i) and N_(o),respectively, an impedance of the inner coil is Z_(i), and a maximumvalue and a minimum value of a combined impedance of the outer coil andthe outer capacitor obtained by varying an electrostatic capacitance ofthe outer capacitor are Z_(o)(_(max)) and Z_(o)(_(min)), respectively,the following inequation is established:|N _(o) /Z _(o(max)) |<|N _(i) /Z _(i) |<|N _(o) /Z _(o(min))|
 13. Theplasma processing apparatus of claim 12, wherein each of the inner coiland the outer coil has a circular ring shape, and if radiuses of theinner coil and the outer coil are represented as “r_(i)” and “r_(o)”,respectively, a radius of a wire thickness of each of the coils isrepresented as “a”, a vacuum permeability is represented as “μ_(o)”, andwhen the combined impedance of the outer coil and the outer capacitorbecomes the maximum value Z_(o)(_(max)) and the minimum valueZ_(o)(_(min)), an electrostatic capacitance of the outer capacitor isrepresented as “C_(o)(_(max))” and “C_(o)(_(min))”, respectively, thefollowing inequation is established:$\frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\max)}}}} < \frac{1}{2\pi \; {fr}_{i}{\mu_{0}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} < \frac{1}{{2\pi \; {fr}_{o}{\mu_{0}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{o}C_{o{(\min)}}}}$14. A plasma processing apparatus, comprising: a processing chamberhaving a dielectric window; a substrate holding unit for holding thereona processing target substrate within the processing chamber; aprocessing gas supply unit configured to supply a processing gas intothe processing chamber in order to perform a plasma process on theprocessing target substrate; an RF antenna provided outside thedielectric window in order to generate plasma of the processing gaswithin the processing chamber by inductive coupling; and a highfrequency power supply unit configured to supply a high frequency powerhaving a frequency for generating a high frequency electric discharge ofthe processing gas in the RF antenna, wherein the RF antenna include aninner coil, an intermediate coil, and an outer coil with gapstherebetween in a radial direction, respectively, and the inner coil,the intermediate coil and the outer coil are electrically connected toone another in parallel between a first node and a second node providedin high frequency transmission lines of the high frequency power supplyunit, and a fixed or semi-fixed inner inductor, a variable intermediatecapacitor, and a variable outer capacitor are provided between the firstnode and the second node, and the fixed or semi-fixed inner inductor iselectrically connected to the inner coil, the variable intermediatecapacitor is electrically connected in series to the intermediate coil,and the variable outer capacitor is electrically connected in series tothe outer coil.
 15. A plasma processing apparatus comprising: aprocessing chamber having a dielectric window; a substrate holding unitfor holding thereon a processing target substrate within the processingchamber; a processing gas supply unit configured to supply a processinggas into the processing chamber in order to perform a plasma process onthe processing target substrate; an RF antenna provided outside thedielectric window in order to generate plasma of the processing gaswithin the processing chamber by inductive coupling; and a highfrequency power supply unit configured to supply a high frequency powerhaving a frequency for generating a high frequency electric discharge ofthe processing gas in the RF antenna, wherein the RF antenna includes aninner coil, an intermediate coil and an outer coil with gapstherebetween in a radial direction, respectively, and the inner coil,the intermediate coil and the outer coil are electrically connected toone another in parallel between a first node and a second node providedin high frequency transmission lines of the high frequency power supplyunit, and a variable inner capacitor, a variable intermediate capacitor,and a fixed or semi-fixed outer capacitor are provided between the firstnode and the second node, and the variable inner capacitor iselectrically connected to the inner coil, the variable intermediatecapacitor is electrically connected in series to the intermediate coil,and the fixed or semi-fixed outer capacitor is electrically connected inseries to the outer coil.
 16. The plasma processing apparatus of claim15, wherein if the number of windings of the inner coil and the numberof windings of the outer coil are N_(i) and N_(o), respectively, animpedance of the outer coil is Z_(o), and a maximum value and a minimumvalue of a combined impedance of the inner coil and the inner capacitorobtained by varying an electrostatic capacitance of the inner capacitorare Z_(i)(_(max)) and Z_(i)(_(min)) respectively, the followinginequation is established:|N _(i) /Z _(i(max)) <|N ₀ /Z _(o) |<|N _(i) /Z _(i(min))|
 17. Theplasma processing apparatus of claim 16, wherein each of the inner coiland the outer coil has a circular ring shape, and if radiuses of theinner coil and the outer coil are represented as “r_(i)” and “r_(o)”,respectively, a radius of a wire thickness of each of the coils isrepresented as “a”, a vacuum permeability is represented as “μ_(o)”, andwhen the combined impedance of the inner coil and the inner capacitorbecomes the maximum value Z_(i)(_(max)) and the minimum valueZ_(i)(_(min)), an electrostatic capacitance of the inner capacitor isrepresented as C_(i)(_(max)) and C_(i)(_(min)), respectively, thefollowing inequation is established:$\frac{1}{{2\pi \; {fr}_{i}{\mu_{o}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{i}C_{i{(\max)}}}} < \frac{1}{2\pi \; {fr}_{o}{\mu_{o}\left( {{\log \frac{8\; r_{o}}{a}} - 2} \right)}} < \frac{1}{{2\pi \; {fr}_{i}{\mu_{o}\left( {{\log \frac{8\; r_{i}}{a}} - 2} \right)}} - \frac{1}{2\pi \; {fN}_{i}C_{i{(\min)}}}}$18. The plasma processing apparatus of claim 1, further comprising: anoutput common capacitor connected between the second node and a groundpotential at an output side.
 19. A plasma processing method forperforming a plasma process on a processing target substrate by using aplasma processing apparatus having a processing chamber having adielectric window; a substrate holding unit for holding thereon aprocessing target substrate within the processing chamber; a processinggas supply unit configured to supply a processing gas into theprocessing chamber in order to perform a plasma process on theprocessing target substrate; an RF antenna provided outside thedielectric window in order to generate plasma of the processing gaswithin the processing chamber by inductive coupling; and a highfrequency power supply unit configured to supply a high frequency powerhaving a frequency for generating a high frequency electric discharge ofthe processing gas in the RF antenna, the plasma processing methodcomprising: segmenting the RF antenna into an inner coil, anintermediate coil, and an outer coil with gaps therebetween in a radialdirection, respectively, the inner coil, the intermediate coil and theouter coil being electrically connected to one another in parallelbetween a first node and a second node provided in high frequencytransmission lines of the high frequency power supply unit; providing avariable intermediate capacitor and a variable outer capacitor betweenthe first node and the second node, the variable intermediate capacitorbeing electrically connected in series to the intermediate coil, thevariable outer capacitor being electrically connected in series to theouter coil, no reactance device being connected to the inner coil; andcontrolling plasma density distribution on the processing targetsubstrate by selecting or variably adjusting electrostatic capacitancesof the intermediate capacitor and the outer capacitor.
 20. The plasmaprocessing method of claim 19, wherein the current flowing in the innercoil is reduced by making at least one of electrostatic capacitances ofthe intermediate capacitor and the outer capacitor close to a value whena series resonance is generated.
 21. The plasma processing method ofclaim 19, wherein the current flowing in the inner coil is increased bymaking at least one of electrostatic capacitances of the intermediatecapacitor and the outer capacitor apart from a value when a seriesresonance is generated.
 22. The plasma processing method of claim 19,wherein electrostatic capacitances of the intermediate capacitor and theouter capacitor are adjusted to enable plasma density on the processingtarget substrate to be uniformized in a diametrical direction.
 23. Theplasma processing method of claim 19, wherein the plasma density on theprocessing target substrate is adjusted so as to be uniformized in thediametrical direction by conforming multiplication of the number ofturns of each of the intermediate coil and the outer coil and an amountof a coil current thereof to multiplication of the number of turns ofthe inner coil and an amount of a coil current thereof.
 24. The plasmaprocessing method of claim 19, wherein a direction of the currentflowing in the intermediate coil is opposite to a direction of thecurrent flowing in the inner coil in the circumferential direction byvariably adjusting the electrostatic capacitance of the intermediatecapacitor in a range smaller than a value when the intermediatecapacitor and the intermediate coil generate a series resonance.
 25. Theplasma processing method of claim 19, wherein a direction of the currentflowing in the intermediate coil is identical to a direction of thecurrent flowing in the inner coil in the circumferential direction byvariably adjusting the electrostatic capacitance of the intermediatecapacitor in a range larger than a value when the intermediate capacitorand the intermediate coil generate a series resonance.
 26. The plasmaprocessing method of claim 19, wherein a direction of the currentflowing in the outer coil is opposite to a direction of the currentflowing in the inner coil in the circumferential direction by variablyadjusting the electrostatic capacitance of the outer capacitor in arange smaller than a value when the outer capacitor and the outer coilgenerate a series resonance.
 27. The plasma processing method of claim19, wherein a direction of the current flowing in the outer coil isidentical to a direction of the current flowing in the inner coil in thecircumferential direction by variably adjusting the electrostaticcapacitance of the outer capacitor in a range larger than a value whenthe outer capacitor and the outer coil generate a series resonance.